1. Field of the Invention
The invention relates to apparatus for operating low temperature electronic devices, and more particularly, for cooling such devices to the necessary temperature.
2. Description of Related Art
In order to use superconducting technologies to measure electrical waveforms produced by room temperature devices, or indeed to interface any low temperature electronic device to a room temperature electronic device, an interface scheme must be found which satisfies the electrical, mechanical, and temperature constraints discussed below:
Electrical Constraints.
when operating at high frequencies and extremely short pulse durations, any power lost in the transmission line between the low temperature circuit and the room temperature circuit will degrade the signal transfer. This degradation appears as pulse dispersion or pulse spreading. To minimize loss, the transmission lines should be made of a low resistance material, be as short as possible, and have the largest possible cross sectional area. The latter constraint is limited by the further constraint that the width of the transmission line should not exceed the wavelength of the maximum frequency of interest, because larger conductors will waveguide and cause geometric losses.
Mechanical Constraints.
Since one end of the transmission line will be operating at extremely low temperatures and the other end will be operating at room temperatures, it is important that the transmission line be able to withstand that temperature difference. Thus, the bond between the transmission line and the low temperature device should be able to withstand that low temperature, and the seal through which the transmission line passes between the low temperature volume and the room temperature volume should also be able to withstand the necessary temperatures. These elements should also be able to withstand repeated cycling from room temperature to low temperature for maintenance, replenishment of helium supply, and general everyday use. Additionally, the temperature coefficient of expansion of the transmission lines should closely match that of the low temperature device, and the construction should be such as to permit the apparatus to tolerate vibration and temperature-induced changes in transmission line length (collectively referred to herein as "movement").
Temperature Constraints.
In order to prevent extensive heat transfer from the room temperature volume to the low temperature volume, the transmission lines should be as long as possible. This is directly contrary to the electrical constraints which favor short transmission lines. The transmission lines should also be made of a material which has low thermal conductivity. Since low thermal conductivity usually implies low electrical conductivity, this constraint, too, is contrary to the electrical constraints.
Workers in the field of superconducting electronics typically achieve the necessary temperatures by immersing their circuits in liquid helium. See, for example, Hamilton, "High-Speed, Low-Crosstalk Chip Holder for Josephson Integrated Circuits," IEEE Trans. on Instrumentation and Measurement, Vol. IM-31, pp. 129-131 (1982). The arrangement shown therein involves attaching several coaxial cables to a Josephson Junction chip which is to be immersed in a liquid helium dewar. See also Hamilton et al., IEEE Transactions on Magnetics, Vol. MAG-17, pp. 577-582 (1981), in which a low-temperature chip is inserted partially inside a coaxial line to couple the signals therethrough to the room-temperature devices. Although not mentioned in the reference, it is believed that the low-temperature chip is then immersed in liquid helium. Both arrangements are constrained to have large coaxial lines which have high thermal conductivity. In order to avoid heat losses, the lines are therefore constrained to be long. In addition, these arrangements cannot be adapted easily to planar chips. Furthermore, at least the latter system is constrained to couple only one line to a chip, which limits the system in utility.
An attempt to deal with the constraints described above appears in U.S. Pat. No. 4,498,046 to Faris. The interface described therein includes a pass-through liquid-helium-tight vacuum seal which consists of a flange and two half-cylindrical fused quartz portions, unequal in length, which act as a pass-through plug from a liquid-helium filled cryostat to a vacuum chamber. Fused quartz, while thermally non-conductive, forms a low loss dielectric substrate for conductive copper striplines which are patterned on the flat surface of the longer portion. The coefficient of expansion of fused quartz is small and relatively well matched to that of silicon, which is used for Josephson and semiconductor chip substrates.
The two fused quartz half-cylinder portions of the pass-through plug are arranged so that the portion with the copper striplines extends sufficiently beyond its mating half-cylinder portion on both ends to provide two platforms at opposite ends of the plug. The low temperature semiconductor chip or device is mounted on one of these platforms and the room temperature chip or device is mounted on the other. The cylindrical geometry was chosen in order to minimize stress on cement used to seal the chamber wall around the pass-through. The planar nature of the striplines allows low inductance connections to be made directly to the two chips which are also planar. The low inductance contacts are copper spheres or other rigid probes, about 100 um in diameter or smaller, which penetrate solder pads on the chips when forced into contact by mechanical pressure. The wall of the cryostat is sealed around the pass-through with a thin layer of non-conductive cement. In operation, the two chips are mounted on the platforms and the pass-through is inserted through the cryostat wall such that the low temperature chip is immersed in liquid helium in the cryostat and the room temperature chip is disposed inside the vacuum chamber. A heating element and thermocouple are placed near the position of the room temperature chip in order to warm it. This chamber must be evacuated in order to prevent frosting of water and other gases on the plug, and also to provide adequate insulation for the cryostat.
The '046 apparatus has numerous problems which render it costly, unreliable and impractical to use in most applications. First, the only method described in the '046 patent for cooling the low temperature device involved immersing it in liquid helium. This renders the apparatus bulky and cumbersome.
Second, the apparatus requires at least two seals, one between the cryostat and the vacuum chamber, and one between the vacuum chamber and the external environment. At least the first of these seals is extremely difficult to create, because it must operate at cryogenic temperatures, must be able to be cycled many times between cryogenic and room temperatures, and must be able to withstand a certain amount of vibration without breaking. Due to the small size of the helium atom, it can pass through extremely small cracks in the seal and can even pass through most materials which are not cracked. This severely limits the types of seals which can be used.
Third, since the low temperature chip is fabricated on a silicon substrate and the transmission line is fabricated on a fused quartz substrate, the two elements must usually be made separately and then mechanically and electrically bonded together. These additional steps are costly. In addition, even though their respective temperature coefficients of expansion are close, the mere fact that the materials are different requires some mismatch which degrades the electrical connection and the mechanical reliability of the bond.
Fourth, because multiple sealed layers of chambers and insulating material are required, the transmission line which carries electrical signals between the two chips must be very long.
Fifth, the pass-through of the '046 apparatus has to be cylindrical in order to obtain a good seal. This renders it difficult to manufacture, and requires special geometries such as that shown in FIG. 3E of the '046 patent.
Finally, the chips used in the '046 apparatus cannot be easily plugged in or out in order to change them.
As the above examples suggest, the field of low temperature electronics appears to suffer from a presumption that immersion in liquid helium is the only feasible method of achieving the necessary temperatures. In the field of optics, devices are sometimes cooled using a product known by the trademark Heli-Tran, made by Air Products and Chemicals, Allentown, PA. The Heli-Tran comprises a vacuum enclosed mounting head for holding the sample to be cooled, and a multi-channel flexible transfer tube for connecting the mounting head to a dewar of liquid helium. Although the construction of the transfer tube is not entirely clear, it is believed to comprise a forward helium flow capillary (from the dewar to the mounting head), a shield tube surrounding the forward helium flow capillary, and a separate return flow capillary for the shield fluid. When the dewar is pressurized, liquid helium flows through both the forward helium flow capillary and the shield tube into the mounting head. The helium in the capillary strikes the inside surface of a metal block closing off the end of the transfer tube, then enters a passage coaxially surrounding all the transfer tube elements, travels a short distance in the return direction, and exits through a helium exhaust port. The helium in the shield tube turns back before the metal block, enters the return flow capillary, and exits from a shield flow return port near the dewar. The sample holder is attached to the outside of the metal block, so that it can conduct heat from the sample to be cooled into the metal block, which is itself cooled by the helium in the forward flow capillary.
A primary drawback with the Heli-Tran system is that the mounting head is entirely enclosed in a vacuum shroud, rendering sample demounting difficult and cumbersome. This drawback is accentuated by the large number of small parts associated with the mounting head which must be removed and reinstalled when a sample is replaced. Additionally, with respect to low temperature electronic circuits specifically, the metal block and sample holder are typically much larger than the circuit itself. A significant amount of helium is therefore consumed for cooling thermal mass which does not itself need to be cold. Moreover, the literature teaches total immersion of a superconducting electronic circuit even in combination with the Heli-Tran system or an apparent variation thereof. In U.S. Pat. No. 3,894,403 to Longsworth, FIG. 5, such a system is shown cooling a liquid helium bath in which a superconducting magnet is totally immersed.
Since total immersion structures make little or no effort to limit the immersed surface area, the consumption of liquid helium typically is very large. The present invention derives in part from the observation that the consumption of liquid helium can be significantly reduced if only the region in which the low temperature circuit is located is cooled.